Generally, in a commercial electrostatographic reproduction apparatus (such as copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged photoconductive or dielectric member. Pigmented marking particles (toner) are attracted to the latent image charge pattern to develop such image on the dielectric member. A receiver member, such as paper, is then brought into contact with the dielectric member and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric member and the image is fixed or fused to the receiver member by heat and/or pressure to form a permanent reproduction thereon.
Sometimes copies made in Xerographic or electrostatic imaging systems have defects caused by improper fusing of the marking material or toner to the receiving media such as paper. There can be many possible causes of these defects including toner contaminants, charging problems or incomplete fusing. In these systems, the image is fixed to the receiving member by heat and pressure to form a permanent reproduction thereon.
Typical electrographic reproduction apparatus includes at least one heated roller having an aluminum core and an elastomeric cover layer, and at least one pressure roller in nip relation with the heated roller. The fusing device rollers are rotated to transport a receiver member bearing a marking particle image, through the nip between the rollers. The toner or pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacking marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member. Accordingly, upon cooling, the marking particle image is permanently fixed to the receiver member.
In some instances, low melting point marking particles or toner are subject to increase image offset to the heating roller. This can produce streaks and undesirable defects in the final copy. This image offset can be reduced by application of fusing oil to the heating roller. The use of such oil introduces further complications into the fusing system, such as handling of the oil and making sure that the layer of oil on the roller is uniform. Alternatively, a mechanical arrangement for reducing image offset, without the need for fusing oil has been found. Such mechanical arrangement provides an elongated web which is heated to melt the marking particles and then cooled to cool the particles and facilitate ready separation of the receiver member with the marking particle image fixed thereto from the elongated web. The nature of operation of the elongated web arrangement also serves to increase the glossiness of the fixed marking particle image. As a result, such arrangement is particularly useful for multi-color image fusing, but is not particularly suitable for black image fusing. There is a need for a convenient and effective way to improve the fusing operations in Xerographic systems.
An option that does not involve core fuser redesign is the preheat concept. By heating the media or paper and toner prior to entering the conventional fusing nip, it is possible to increase productivity for a given fusing system, or if desired, trading speed benefits for lower temperature and/or wider media latitude at current throughputs. After considerable study, it was decided to pursue hot air impingement as the preferable means of preheating . Radiant heating was eliminated due to the risk of fire. Conduction was not used since touching unfused prints typically lead to image defects. Convective heating is much safer than radiant and much more “image-friendly” than conduction. This invention introduces a design concept that directly addresses the fundamental issues of heat transfer efficiency, uniformity and paper handling.